This invention relates generally to magnetic particle displays and particularly to apparatus and a method for individually encapsulating magnetic particles for use in such displays. Magnetic particle displays are typically flat panel, matrix addressable display devices. The displays form images on a panel of freely rotating spherical particles, each of which is a tiny permanent magnet, dark colored in one hemisphere and light colored in the other. Thus, the amount of ambient light reflected by the particles is a function of the particle orientation which is controlled by a magnetic field. Since the magnetic particles are generally spherical as opposed to disk shaped, the particles do not need to be pivoted for rotation. It is then practical to use very small particles on the order of 1 millimeter (mm) or less in diameter or linear dimension and in very large numbers. The magnetic particles are typically smaller than can be resolved by the naked eye thus giving the display a high resolution.
Fabrication of a magnetic particle display requires combined efforts in four rather unrelated technological areas. First, one must make the spherical particle. Second, one must impart to these particles the desired optical and magnetic properties. Third, the particles must be encapsulated for positioning on the surface on which the image is to be produced; and finally, a magnetic field must be provided to control the orientation of the encapsulated particles. The method and apparatus of the present invention are concerned with and are directed to the foregoing noted third step of fabrication wherein the particles are encapsulated for placement within the environment wherein the image is to be produced. More particularly, a method is needed to encapsulate individual ones of the extremely small particles within a carrier fluid medium for rotatable installation within the display. One of the more difficult problems involved in encapsulation is the dispersal of a large number of agglomerated magnetized spherical particles in such a manner that individual ones of the particles can be separately and uniquely placed within associated ones of the capsules. Since the particles are magnetized, they tend to attract each other due to the inherent magnetic forces and thus resist separation and dispersal for placement into individual capsules. Furthermore, surface tension of the surrounding liquid prevents the particles from being separated. In other words, the interfacial tension of the oil and water interface makes it difficult for larger oil drops to separate into smaller ones. The surface tension force can be characterized as a short range force that generally operates only when the particles are in very close proximity to each other and is a relatively strong force to overcome. Thus, when the particles are so close to each other that the surrounding oil forms a continuous volume, there is usually a relatively strong force to overcome. The magnetic force, in contrast, can be characterized as a long range force that tends to pull particles together from greater distances and is a relatively weak force, especially at large distances.
One method known in the prior art for providing dispersal of the agglomerated magnetized particles is the use of mechanical agitation devices which interact with and disperse the agglomerated particles when such particles are placed in a carrier fluid such as oil. In such a method, the degree to which the dispersal is accomplished largely depends on the intensity of the applied mechanical forces with the greater applied mechanical forces resulting in the greater dispersal but also with the concurrent possibility of removing all the oil surrounding the particles. The implementation of such a method requires a certain delicacy and sensitivity in impacting the particles with the agitating means so as to create a reasonable yield of oil covered useful particles.